In recent years, there has been an increasing trend towards making timepieces of thinner and more attractive shape. This is particularly true in the case of timepieces of the so-called analog type, i.e. using time indicating hands. However one difficulty which has been encountered in producing such timepieces has been that of manufacturing timepiece hands which are very thin, yet which should also be strong and rigid, accurately shaped, and which can be firmly attached to the hands driving shafts of the timepiece.
One method which has been used in the prior art to produce timepiece hands of small size has been to press-cut, i.e. to stamp out the hands from a sheet of metal, and then to form the upper face of the hands into a desired shape by machining with a diamond tipped cutting tool. However this method of manufacturing has several disadvantages in the case of timepiece hands of very thin shape. One of these disadvantages is that, as a result of the machining with a diamond tipped cutting tool, bending distortion of the hand will be caused, and the amount of this distortion will increase as the thickness of the hand is reduced. Another disadvantage of this manufacturing method is that, since the thickness of the boss of the hand cannot be very much greater than that of the rest of the hand (i.e. than the thickness of the sheet metal from which the hands were stamped), it is difficult to obtain a sufficient degree of attachment force between the hand shaft of the timepiece and the bore in the boss of the timepiece hand. This problem becomes increasingly severe as the thickness of the timepiece hand is reduced.
Another method used to produce small size timepiece hands is the electro-forming manufacturing method. With this method, electro-forming of a number of layers of different metals is performed on a suitable matrix, with the shape of a portion of the matrix determining the size and shape of the final timepiece hand. However, as will be described later with reference to one of the drawings, it is necessary to perform grinding work upon the matrix in this case, in order to uncover the bore of the timepiece hand boss, before removal of the timepiece hand from the electro-forming matrix. During this grinding work, stresses are introduced into the multiple layers of metal of the timepiece hand, which will result in bending distortion of the hand when it is removed from the matrix. The amount of this bending distortion increases as the thickness of the hand is reduced. In addition, a considerable length of time and a complex electro-forming process are required to form the various metal layers which constitute the timepiece hand, making the manufacturing production costs very high. Thus, such a prior art type of electro-forming process is only used for hands to be provided in very high quality timepieces.
The present invention proposes a method of manufacturing very thin and small timepiece hands, by an electro-forming process, whereby the completed hand is in the form of an open shell of hard nickel electro-forming which is extremely strong and light in weight, and whereby bending distortion of the completed hand is completely eliminated, even when the hand is made of very thin shape. With this method, the electro-forming process can be completed in a small fraction of the time required by a prior art method, and the completed timepiece hands can be rapidly and easily removed from the electro-forming matrix. The method therefore enables timepiece hands of very thin shape to be produced at a low level of manufacturing cost, so that the applications of such hands are not restricted to very expensive timepieces.